| Igneous Rocks |
These rocks are formed from the solidification of magma and tends to
contain magnetite. As temperature of lava drops below the Curie point
(600 degrees Celsius), the magnetite particles orient along earth's
magnetic field. As rock solidifies, and its particles are not reoriented
by further changes in the earth's magnetic field. |
| Magnetic Dip |
Magnetite is not only oriented north and south, but also at an angle to
the horizon depending on the latitude. The dip angle is 0 degrees at
Equator and 90 degrees at poles. The dip is the latitude of the rock
when it is first formed. |
| Apparent Pole Wandering |
The history of the rock is also known as pole wandering. The wandering
paths deduced from rocks of different continents diverged, unless
continents moved. |
| Magnetic Pole Reversals |
The poles reverse sign 1 to 2 times every million years. A reversal
lasts a few thousand years. Presently, the magnetic field is weakening
and will disappear in 2000 years. |
| Fossil Dating |
Compare assemblages of fossils in sediments. |
| Relative Dating |
The absolute date of the fossil is not known. Fossils go back to 600 Ma,
but the earth goes back to 4600 Ma. |
| Radiometric Dating |
By measuring the ratio of radioactive isotope (e.g. U235) to its decay
product (Pb207), and knowing the half-life h, you can then determine the
age T. e.g. in a sample, ratio of U235 to Pb207 is 1: 3 => only 1/4 of
the original U235 remains. At T = h, 1/2 of U235 remains; at T = 2h, 1/4
remains. Thus T = 2h = 1400 Ma. |
| Fossil and Glacial
Evidence | Presence of ancient tropical coral assemblage,
polar and tropical flora in sediments gives clue to where the land mass
was located in ancient times. |
